This invention relates to a method and apparatus for a multistage pressure control of an injection molding machine during a holding step thereof after filling a molten resin in the mold cavity of the machine, and more particularly a method and apparatus for transferring the pressure transfer points in respective holding steps at points when the resin temperature or the mold temperature reaches a predetermined preset temperature by means of a temperature sensor mounted on the mold.
According to a prior art method of controlling a pressure holding step, the step is time divisioned into multistages and pressures in respective stages are preset so that products having small through large thicknesses can be obtained which are free from shrinkage and have satisfactory appearance. However, dependent upon the type of resin used, the temperature at which the molten resin solidifies becomes different. Further, the time required for the molten resin to solidify becomes different dependent upon such factors as the volume of the molded product the temperature of the metal mold, so that the operator should have many experiences in order to set desired times and pressures.
Especially, where a high holding pressure is applied to the molten resin while it is solidifying, a large internal stress would be created in the molded product, thus affecting the mechanical strength thereof. In a precise molded product such as a compact disc it is necessary to obtain a molded product having a good appearance, a high transfer printing property and a low stress so that it is impossible to use a high holding pressure. In such product it is necessary to set the holding pressure by taking into consideration not only the operator's experience but also the influences of the external temperature and the resin characteristics.